Polyvinyl chloride (PVC) polymers are used in large volumes throughout the world to make various end use products. These polyvinyl chloride resins are normally produced and sold in the form of a free flowing powder made up of individual resin particles. The resin particles are then extruded or calendered into various shapes to form useful products such as pipe and house siding.
An important feature of the resin in extrusion operations is the shape of the resin particle. Assuming a constant particle size and porosity, the more spherical the shape of the resin particle the faster the extrusion rate. Also, a spherical shaped particle will give higher bulk density which results in savings in shipping costs. The higher the bulk density, the more pounds can be loaded into the same volume railcar. Current commercial resins vary in shape from a highly irregular shape resembling "popcorn" to irregular eliptical shapes resembling a distorted "egg".
Producers of polyvinyl chloride have long sought to produce a spherical shape particle. When this desirable goal was attempted in the past, producers found other problems which made the particle undesirable for use in extrudates. One of the problems encountered was what is referred to in the art as particles having "glassy" content or "clears" as they are sometimes referred to in literature. A particle having glassy content sometimes has a transparent appearance and will not readily absorb plasticizer. Sometimes substantial amounts of glassy content can be present as a portion of non-glassy looking particles and, therefore, might not be seen microscopically. This is an important distinction in that some resins might be reported as having low amounts of glassy or clear particles, but in fact, the glassy content of the particles could be much higher. Also residual monomer is difficult to strip from glassy portions of resin. When extruded, resin having large amounts of glassy content will not properly fuse and mix with other particles thus leading to weak spots in the extrudate. The glassy content acts somewhat like contamination by foreign objects. The result of the presence of large amounts of glassy content, of course, is an extrudate with poor impact resistance.
U.S. Pat. Nos. 3,620,988, 3,706,722, 4,229,547, 4,360,651, and 4,458,057 all disclose attempts to produce polyvinyl chloride resin having spherical shaped particles. However, a resin having spherical shaped particles and having low amounts of glassy content was unknown before this invention.
It is a long sought after and desirable goal of the polyvinyl chloride industry to have a resin with spherical shaped particles and low amounts of glassy content.